The present invention relates to a method for making a cylindrical screening device whereby filtration takes place through narrow slot openings formed between the successive windings of juxtaposed helically wrapped elements. It is generally known to make slot screens, in particular cylindrical slot screens, hereinafter called slot tubes, by welding a helically wound wire (forming the cylindrical screening surface and hereinafter called screening wire) in successive contact points to supporting wires longitudinally extending according to the generating line of the cylinder. The screening wire often is a profile wire having for example a triangular cross-section. In order to ensure strict tolerances on the size of the slot opening, it is of utmost importance that the heli formed by the screening wire have a perfectly cylindrical shape. It has already been proposed to pull the screening wire, which is fed to the welding station, between a set of unpowered bending rollers located close to the welding station in order to give the screening wire the desired curvature.
However, this method proved to be ineffective since the curved screening wire segment located between the welding station and the bending rollers excessively straightened so that the ultimately desired cylindrical curvature of the screening wire spiral was hard to control. As a remedy, it was then proposed to feed the aforementioned curved screening wire segment tensionless to the welding station by driving a set of bending rollers in a suitable manner and synchronizing the driving speed of the set of bending rollers to that of the forward motion of the screening wire through the welding device. This technical solution, however, has the disadvantage of requiring an additional synchronization device on each welding machine. Moreover, the bending device must always be located outside the circumference of the well screen so that, in the curved wire segment between the bending device and the welding station, forces are inevitably caused to exist, which counteract somewhat the applied curvature (greater curvature radius in said segment than that applied in the bending device).
The present invention now provides means to obviate the above mentioned disadvantages. Therefore the method according to the invention essentially comprises two process steps. In a first process step the screening wire is continuously bent between a set of bending rollers into a helical wrap with juxtaposed equal windings and with a diameter substantially equal to the desired ultimate diameter of the well screen. The radius of the curvature, which is being applied in the wire in this process, is essentially smaller than the diameter of the obtained helical wrap in order to allow for the elastic extension of the bent shape.
In a second process step, the preformed cylindrical helical wrap is fed to the welding station to be connected to the supporting wires, which extend according to the generating line of the cylinder, by means of a known welding method.
The preliminary bending of the wire into the cylindrical wire wrap with a predetermined diameter is a simple process that can be carried out on a bending device operating independently of the welding device, so that synchronization is not necessary. Bending may also be done much faster than welding so that one bending device may suffice for feeding several welding machines.
The separate bending of the screening wire also permits to carry out the bending operation in an area located inside the outer circumference of the windings. The curved wire leaving this bending area then extends to the desired screen diameter while forming the desire helical wrap.
The progressive guiding and bringing into contact of successive segments of the thus preformed helical wire in the welding device is also done without applying tension on the preformed spiral, as will be described further on.